1. Field of the Invention
The present invention generally relates to calibration methods, and more particularly to a method to automatically diagnose photometric particle analyzers.
2. Discussion of the Background
There is a class of instruments, often referred to as “photometric particle analyzers” that analyze airborne particles by collecting samples of the particles on a filter, and then sensing the collected particles on the filter. One example of such an instrument is an optical photometric particle analyzer known as an AETHALOMETER® (Magee Scientific Corporation, Berkeley, Calif.). The AETHALOMETER® collects particles suspended in an air stream by continuous filtration of the air through a porous, fibrous substrate, such as a quartz fiber filter. Simultaneously, the optical transmission of the filter is continuously measured by illumination of the upper surface, and detection of the light transmitted through various portions of the filter to its underside.
If the sampled air stream of an AETHALOMETER® contains optically-absorbing particles, the accumulation on the filter will contain a mass of material (including the “Black Carbon” component, denoted “BC”) that will absorb some of the incident light, and reduce the amount of light transmitted through the filter. The AETHALOMETER® precisely measures the intensity of light transmitted through the “sensing” portion of filter material, on which particles are collected, and compares this with the intensity of light emanating from the same source but passing through a “reference” portion of the same filter material but which is not subject to the collection of particles. The AETHALOMETER® may function, for example, by using photo-detectors to accurately measure the intensity of light transmitted through various portions of an “optical analysis head.” The illuminating light is provided by a multiplicity of light-emitting diode (LED) sources emitting at a variety of wavelengths. The ratio of these intensities leads to the calculation of optical absorption, since the common denominator is the intensity of light produced by the source. The relation between optical absorption and mass of BC, determined by separate laboratory techniques, allows the increase in optical absorption between one measurement and the next to be interpreted as the accumulation of a certain amount of BC during the measurement period. Since this material was collected from the flowing air stream, the concentration of BC in the sample air stream can be calculated, given a simultaneous measurement of the air flow rate by a separate sensor. In this way, the AETHALOMETER® offers a real-time measurement of the concentration of BC particles in an air stream.
While the use of photometric particle analyzers is effective, and generally have a linear response, variations of light intensity or detector response can degrade the accuracy of the instruments. There are many different potential sources of error in the use of photometric particle analyzer. Thus, for example, degradation in accuracy can result from contamination of the optics or the light sources, or from changes in the linearity of response of the instrument. There is a need in the art for a method of determining when the performance of a photometric particle analyzer is degraded under such circumstances. Such a method should be compatible with existing instruments, should be easy to implement.